I'm finally implementing the advice given in this question, I've managed to run the wiring up to the junction box, but I'm a bit confused by the contents of that box.
There are three circuits entering the box, but it appears that two circuits are sharing a single neutral.
Everything appears to work (and has done so for 2+ years), but I'm curious if this is kosher from a code point of view. Should there be an individual neutral for each circuit? If so, is there a brief explanation as to why this is OK?